In the
figure below, when nothing is holding object A, object A falls
to the floor. However, object B over the table does not fall to the
floor. What does the table do to object B? As we discussed previously,
for all objects near the surface of the Earth, the
weight of the object
represents the attractive force acting over the objects. Thus, over objects A
and B a force, corresponding to their respective weights, pulls
the objects down. Object A falls because wA is the
only force acting on the object; therefore, the net force acting on object A
is

However, object B is not accelerating implying that the net force
acting on it must be zero,
.
What is holding object B is the table; therefore, the table must
apply a force on object B equal and opposite to the weight of the
object. This force is called the normal force.
Why does the table applies a force over the object? The object is pushing
down on the table with a force equals to the weight of the object (action);
then, the table reacts by applying a force of the same magnitude but
opposite direction to the object (reaction).

Characteristics of the Normal Force.

The normal force is the reaction of the surface to the action or push of
the object over the surface.

The magnitude of the normal force is equal to the magnitude of the push
against the surface exerted by the object. If the surface is horizontal, the
magnitude of the normal force is equal to the weight of the object.

The direction of the normal force is alwaysperpendicular to the surface and points
out of the surface.

On the figure at the right, the unbalance force makes
the object to slide down the inclined. Inclined planes will be study in
details later on this notes. Additionally, the normal force is equal in
magnitude only to the push of the object against the surface. Only
horizontal surfaces produce reaction forces equal to the weight of the
object.

In the case of a man leaning against a wall, the
following forces can be identified:

Weight of the man. Attractive force of the Earth acting on the
man.

Man pushing against the floor. The man push against the floor
with a force equals to his weight.

Normal force floor on man. As a reaction to the man exerting a
force against the floor, the floor reacts by applying a force of the
same magnitude but opposite direction on the man. This force balance
the weight of the man.

Man pushing against wall. By leaning against the wall, the man
exerts a force against the wall, the magnitude of this force depends
on the angle of the legs. The man exerts this force because of the
tendency that the body will have to rotate counterclockwise in this
figure.

Normal force wall on man. As a reaction to the previous force,
the wall exerts a force on the man (perpendicular to the surface).
This force prevents the actual rotation of the man (see
section on equilibrium).

Static Friction on Man. The shoes of the man will have the
tendency to slide forward because of the pushing of the wall on the
man. The static frictional force prevents this for happening. Static
friction is study later in this notes.

Consider a string pulled from the two extremes in opposite direction by a force F,

The string is under the tension
. If the string is pulled from one side
only, the tension on the string is zero. In this case, the force applied to the
string has for result the acceleration of the string

The tension of a string transmit forces from one body to
another. The magnitude of the force transmitted is equal to the magnitude of the
tension. The direction of the force can be understood following the direction of
the action and reaction forces.

Suppose that two blocks are pulling on the ideal string (
it can not stretch and is massless) in opposite direction with forces of the
same magnitude, see below.

The block labeled L pulls on the string
toward the left and the block labeled R pulls on the string
toward the right.

Thus, the tension on the string is
.

For the block labeled L, the block acts
applying a force F to the string
pulling toward the left, the string reacts by applying a force of the
same magnitude but opposite direction (toward the right) over the block,

For the block labeled R, the block acts
applying a force F to the string
pulling it toward the right, the string reacts by applying a force of the
same magnitude but opposite direction (toward the left) over the block,